One of the 7 essential meta questions:
This is rapidly becoming a hot issue across the entire network: how to promote your site and how to reach out to the experts and pundits in your industry. We can come up with budgets and promotions but — more than any other issue raised here — the means and ideas about how to reach your target audience HAS TO come from you and your community. Has to. Has to, has to, has to! We simply are not experts in your field. We don’t have the the connections nor the experience you bring to the table. You are both our evangelist and our ambassador.
A good way to start is to search for questions tagged [promotion] on other metas.
Also, read A Recipe to Promote your Site.